diff options
Diffstat (limited to 'compiler/rustc_mir_transform/src/instcombine.rs')
-rw-r--r-- | compiler/rustc_mir_transform/src/instcombine.rs | 203 |
1 files changed, 203 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/instcombine.rs b/compiler/rustc_mir_transform/src/instcombine.rs new file mode 100644 index 000000000..2f3c65869 --- /dev/null +++ b/compiler/rustc_mir_transform/src/instcombine.rs @@ -0,0 +1,203 @@ +//! Performs various peephole optimizations. + +use crate::MirPass; +use rustc_hir::Mutability; +use rustc_middle::mir::{ + BinOp, Body, Constant, ConstantKind, LocalDecls, Operand, Place, ProjectionElem, Rvalue, + SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp, +}; +use rustc_middle::ty::{self, TyCtxt}; + +pub struct InstCombine; + +impl<'tcx> MirPass<'tcx> for InstCombine { + fn is_enabled(&self, sess: &rustc_session::Session) -> bool { + sess.mir_opt_level() > 0 + } + + fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { + let ctx = InstCombineContext { tcx, local_decls: &body.local_decls }; + for block in body.basic_blocks.as_mut() { + for statement in block.statements.iter_mut() { + match statement.kind { + StatementKind::Assign(box (_place, ref mut rvalue)) => { + ctx.combine_bool_cmp(&statement.source_info, rvalue); + ctx.combine_ref_deref(&statement.source_info, rvalue); + ctx.combine_len(&statement.source_info, rvalue); + } + _ => {} + } + } + + ctx.combine_primitive_clone( + &mut block.terminator.as_mut().unwrap(), + &mut block.statements, + ); + } + } +} + +struct InstCombineContext<'tcx, 'a> { + tcx: TyCtxt<'tcx>, + local_decls: &'a LocalDecls<'tcx>, +} + +impl<'tcx> InstCombineContext<'tcx, '_> { + fn should_combine(&self, source_info: &SourceInfo, rvalue: &Rvalue<'tcx>) -> bool { + self.tcx.consider_optimizing(|| { + format!("InstCombine - Rvalue: {:?} SourceInfo: {:?}", rvalue, source_info) + }) + } + + /// Transform boolean comparisons into logical operations. + fn combine_bool_cmp(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) { + match rvalue { + Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), box (a, b)) => { + let new = match (op, self.try_eval_bool(a), self.try_eval_bool(b)) { + // Transform "Eq(a, true)" ==> "a" + (BinOp::Eq, _, Some(true)) => Some(Rvalue::Use(a.clone())), + + // Transform "Ne(a, false)" ==> "a" + (BinOp::Ne, _, Some(false)) => Some(Rvalue::Use(a.clone())), + + // Transform "Eq(true, b)" ==> "b" + (BinOp::Eq, Some(true), _) => Some(Rvalue::Use(b.clone())), + + // Transform "Ne(false, b)" ==> "b" + (BinOp::Ne, Some(false), _) => Some(Rvalue::Use(b.clone())), + + // Transform "Eq(false, b)" ==> "Not(b)" + (BinOp::Eq, Some(false), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())), + + // Transform "Ne(true, b)" ==> "Not(b)" + (BinOp::Ne, Some(true), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())), + + // Transform "Eq(a, false)" ==> "Not(a)" + (BinOp::Eq, _, Some(false)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())), + + // Transform "Ne(a, true)" ==> "Not(a)" + (BinOp::Ne, _, Some(true)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())), + + _ => None, + }; + + if let Some(new) = new && self.should_combine(source_info, rvalue) { + *rvalue = new; + } + } + + _ => {} + } + } + + fn try_eval_bool(&self, a: &Operand<'_>) -> Option<bool> { + let a = a.constant()?; + if a.literal.ty().is_bool() { a.literal.try_to_bool() } else { None } + } + + /// Transform "&(*a)" ==> "a". + fn combine_ref_deref(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) { + if let Rvalue::Ref(_, _, place) = rvalue { + if let Some((base, ProjectionElem::Deref)) = place.as_ref().last_projection() { + if let ty::Ref(_, _, Mutability::Not) = + base.ty(self.local_decls, self.tcx).ty.kind() + { + // The dereferenced place must have type `&_`, so that we don't copy `&mut _`. + } else { + return; + } + + if !self.should_combine(source_info, rvalue) { + return; + } + + *rvalue = Rvalue::Use(Operand::Copy(Place { + local: base.local, + projection: self.tcx.intern_place_elems(base.projection), + })); + } + } + } + + /// Transform "Len([_; N])" ==> "N". + fn combine_len(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) { + if let Rvalue::Len(ref place) = *rvalue { + let place_ty = place.ty(self.local_decls, self.tcx).ty; + if let ty::Array(_, len) = *place_ty.kind() { + if !self.should_combine(source_info, rvalue) { + return; + } + + let literal = ConstantKind::from_const(len, self.tcx); + let constant = Constant { span: source_info.span, literal, user_ty: None }; + *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant))); + } + } + } + + fn combine_primitive_clone( + &self, + terminator: &mut Terminator<'tcx>, + statements: &mut Vec<Statement<'tcx>>, + ) { + let TerminatorKind::Call { func, args, destination, target, .. } = &mut terminator.kind + else { return }; + + // It's definitely not a clone if there are multiple arguments + if args.len() != 1 { + return; + } + + let Some(destination_block) = *target + else { return }; + + // Only bother looking more if it's easy to know what we're calling + let Some((fn_def_id, fn_substs)) = func.const_fn_def() + else { return }; + + // Clone needs one subst, so we can cheaply rule out other stuff + if fn_substs.len() != 1 { + return; + } + + // These types are easily available from locals, so check that before + // doing DefId lookups to figure out what we're actually calling. + let arg_ty = args[0].ty(self.local_decls, self.tcx); + + let ty::Ref(_region, inner_ty, Mutability::Not) = *arg_ty.kind() + else { return }; + + if !inner_ty.is_trivially_pure_clone_copy() { + return; + } + + let trait_def_id = self.tcx.trait_of_item(fn_def_id); + if trait_def_id.is_none() || trait_def_id != self.tcx.lang_items().clone_trait() { + return; + } + + if !self.tcx.consider_optimizing(|| { + format!( + "InstCombine - Call: {:?} SourceInfo: {:?}", + (fn_def_id, fn_substs), + terminator.source_info + ) + }) { + return; + } + + let Some(arg_place) = args.pop().unwrap().place() + else { return }; + + statements.push(Statement { + source_info: terminator.source_info, + kind: StatementKind::Assign(Box::new(( + *destination, + Rvalue::Use(Operand::Copy( + arg_place.project_deeper(&[ProjectionElem::Deref], self.tcx), + )), + ))), + }); + terminator.kind = TerminatorKind::Goto { target: destination_block }; + } +} |